Reddit, for reasons I'm sure everybody here knows. I do make two exceptions: first, there is one single small community on Reddit that "meets" once a week. I comment on those posts. Second, I'll allow myself to comment on posts about or related to boycotting Reddit.
Google's Android. While I don't currently want to completely switch to another Android OS, that doesn't mean I won't in the future. Currently, though, I do try to modify the OS as much as possible to protect my privacy to a reasonable extent. I recently switched from Chrome to Firefox on both my laptop and phone. I run an adblocker app 24/7 on my phone. And I trusted, modified versions of apps where possible, such as ReVanced. I also recently disabled the Google Play Store and started using the Aurora Store as my default.
I guess you could call these "soft" boycotts.
I would boycott Apple, but I like Android and its ability to install apps from outside their app store way more, and Windows just makes more sense to me than a Mac, so I honestly have no need for Apple.
As for modifying Windows (like I'm doing with Android) or boycotting it, I'm sure that'll happen in time with the way things are going. I've only just recently started looking into stuff like ShutUp10 and other similar stuff, but I want to make sure I know what I'm doing since a Windows OS can be a bit more fragile than an Android OS.